Theoutsideoftheearthisacrustofrockwhichisabout10to30milesthick.Whenwegodownintothiscrust,wefindthatitbeginstogethotterandhotter.Foraboutevery40meterswegodown,thetemperaturegrowsonedegreehigher. 　　Attwomilesbelowthesurfaceoftheearththetemperatureishighenoughtoboilwater!Ifitwerepossibletodigdown30miles,thetemperaturewouldbeabout1,200degreesCentigrade.Thisishotenoughtomeltrocks.Atthecentreoftheearth,scientistsbelievethetemperaturetobeabout5,500degreesCentigrade. 　　Thecrustoftheearthhastwolayers.Theupperlayer,whichmakesthecontinents,ismadeupofgranite.Underthelayerofgraniteisathicklayerofveryhardblackrockcalled"basalt".Thislayersupportsthecontinentsandformsthebasinsthatholdtheoceans.Atthecentreoftheearth,itisbelievedthatthereisahugeballofmolteniron,withadiameterofabout4,000miles. 　　Howdidthecentreoftheearthgettobethisway?Accordingtomostscientifictheories,theearthandsunwereoncerelatedinsomeway.Mostscientistsbelievethattheearthwasonceahot,whirlingmassofgas,liquid,orsolidthatbeganitsregulartripsaroundthesun.Asyearswentby,itslowlycooledandthelargemassgrewsmaller.Asitwhirled,itslowlytookaball-likeshape.Itwasred-hotandheldinitspathbytheattractionofthesun. 　　Astheearthcooled,ahardcrustformedonthesurface.Nobodyknowshowlongittookforthecrusttoform.Butunderneaththatcrustthereremainedthehotcentrecoreoftheearth,anditisstilltheretoday.